Stacking technology connects two or more devices to form a logical device. Users can manage two or more devices by managing a single logical device. Stacking technology can provide high availability and high scalability and simplify management. All devices in a stack use the same configuration file.
Devices in a stack are connected by stack cables. FIG. 1 illustrates a typical stack. As illustrated in this figure, there is an active device in the stack, and others are standby devices. The active device manages the control plane, including executing and applying configurations. The active and standby devices run the data plane and forward data. Data that the standby devices need is applied by the active device.
If the stack cables connecting the active and standby devices are broken, there will be two or more active devices with the same global configuration in the network. When these devices are connected through other links, network failure may occur; for example, IP address collision and Layer 2 protocol computation failure can occur. Therefore, how to detect multi-active collision has become a pending problem of multi-device stacking.
The virtual switching system (VSS) of Cisco allows combination of at most two Catalyst 6500 series switches. VSS uses two mechanisms to detect and solve dual-active collision.
One mechanism is enhanced Port Aggregation Protocol (PAgP). As illustrated in FIG. 2, Cisco has extended the PAgP protocol packets. The new active device carries its ACTIVE_ID in a PAgP packet and sends it to the original active device. If the original active device finds that the ACTIVE_ID in the received PAgP packet is not the same as its local ACTIVE_ID, it considers multi-active collision to have occurred, and then disables all local interfaces. In this way, multi-active collision can be solved.
The other mechanism is Bidirectional Forwarding Detection (BFD). As illustrated in FIG. 3, a respective Layer 3 interface is selected on the two devices in the stack, different IP addresses and static routes are assigned for the two interfaces, and the two interfaces are connected with network cables. When the stack works normally, the BFD session is in a down state; when the stack link fails, the static route on the standby device takes effect, and the BFD session is in an up state, so multi-active collision is detected.
These two mechanisms of Cisco can only support stacking of two devices; that is, they can only implement dual-active detection. When there are more than two devices to be stacked, multi-active detection is needed, and the two mechanisms cannot satisfy the need. As illustrated in FIG. 4, with enhanced PAgP adopted, the original active device is disabled, but collision exists between new active device 1 and new active device 2. The same problem occurs when BFD is deployed.